Cuts down frictions so could cause slippage, reduces the spread on the bar in some cases and so can put it in an unstable positions, puts pressure in unfavorable locations thus actually leading in some cases to the kind of pain he's talking about, limits what position on the traps/delts you can realistically place the bar thus forcing you to adapt a bar placement that may not be the best for you....I'm sure I could think of some more
